Accretion Disks with Mass Evaporation to Corona and their Applications

The principal aim of this thesis is to investigate the properties of an accretion disk with a hot corona above, based on the work of Meyer &Meyer-Hofmeister (1994).First, we study the structure of a classical thin disk using the updated opacity tables and equation of state. Then we take into account for a corona above the thin disk, analyze the physical processes in the corona and compute the structures of the viscosity-heated corona around white dwarfs and around black holes. On the basis of this, we investigate the disk evolution with mass evaporation from the disk to the corona and compare it with the case without corona above the disk. Incorporating an inner advection -dominated accretion flow (ADAF)with the outer thin disk and corona, we show that the disk evaporation could cause switch from an outer thin disk to an inner coronal flow/ADAF. A new view on soft-hard spectral transitions in black hole binaries is also presented based on the disk-corona model. Our scientific results are summarized as follows: In Chapter 2, we replace the opacities used in the calculations of accretion disks by the recently improved opacity tables and the corresponding data of the equation of state (EOS), and compute the vertical structures of accretion disks in cataclysmic variables. The revised viscosity vs. surface density relation is obtained and compared with the old results. Our conclusion is that the improvement on the opacity and EOS hardly influences the disk structure compared to the uncertainties introduced by the parameterization of viscosity and mixing length. In Chapter 3, the corona above a thin disk is investigated in detail. Like in a classical disk, gas in the corona rotates differentially in the gravitational potential well of the central object and meanwhile heat is produced by viscosity. It is the underlying cool disk that causes the distinctly different cooling of gas in the corona. The viscous heat is transported by electron conduction downwards to the transition layer between the corona and the disk, being partially radiated away and partially heating the atmosphere there, thereby leading to continuous mass evaporation from the under lying disk. In such a frame, the structure of the corona is computed by solving the differential equations numerically. We find that the mass evaporation rate is stronger at a smaller distance and reaches the maximum at a certain radius. Such a feature of the evaporation brings important consequences to the disk evolution and transition of different accretion forms, which are intensively investigated in the following chapters. The self-consistence of the disk-corona model is also discussed in this chapter. In Chapter 4, we first investigate the evolution of the accretion disk with a corona above in dwarf nova systems during quiescence. We find that a hole is created in the inner disk region owing to the mass evaporation to the corona, and the size of the hole depends on the mass transfer rate, the viscosity and the initial mass distribution in the disk. The formation of such a coronal hole in the inner disk has at least three advantages over the classical thin disk in interpreting the features observed in dwarf novae, namely, (a)the UV-lag behind the rise of optical flux in outbursts, (b)the post-outburst of X-rays at the beginning of quiescence, and (c)the onset of the outburst, which is usually triggered at a temperature higher than the observed one according to the classical disk model. Secondly, we propose a new scenario for the disk evolution during the quiescence of WZ Sge -type dwarf novae and X-ray novae. These objects exhibit large amplitudes and long outburst recurrence time, in contrast to normal dwarf novae. This constrains the model parameters of the accretion disks and brings difficulties to previous models. A hole created by evaporating the inner disk material prevents a premature outburst. Since a certain amount of angular momentum is continuously brought in by an incoming stream, whereas angular-momentum loss due to the evaporation and tidal action by the companion star is likely to be small in the quiescence, the total angular momentum of the disk increases faster than the mass of the disk. This causes expansion of the disk and accumulation of mass in the outermost part of the disk. The disk may expand to the critical radius for the 3∶1 resonance between Kepler and orbital angular frequencies before an outburst is triggered. Therefore, evaporation changes the evolution of the disk and provides a good explanation for the observational properties of WZ Sge stars, as well as of the X-ray transients. Thirdly, we quantitatively model the evolution of the accretion disk of WZ Sagittae during the long quiescence. We find that the large amount of mass in the disk derived from the outburst luminosity is a severe constraint and demands the value of αc ≈0. 001 in contradiction to some recent suggestions. In the meanwhile, we find a new mode of disk evolution: during quiescence the disk is quasi-stationary, about half of the mass transfered from the companion star flows through the disk and the other half is needed to build up the steadily growing outer disk; when the 3∶1 resonance radius is reached the disk growth ends; from then on all the transferred matter flows inwards and the surface density increases, leading to an outburst within a few years. In Chapter 5 transitions from a thin disk to coronal f lows/ADAFs in a number of well studied black hole binaries are investigated by using the disk evaporation model. The location of this transition is determined by mass accretion rate. By comparing the predicted transition radii with the observed locations of the inner disk edges, which are estimated from the maximum velocity of the Hα emission line, we find that the transition caused by evaporation agrees with the observations. Thus, we propose that the evaporation should be a consistent mechanism of the smooth transfer of accreted gas from an outer thin disk to an inner coronal flow/ADAF. Moreever, the dependence of the transition radius on the accretion rate provides a natural interpretation for the change of spectral states in X-ray binaries. Applications of the model to active galactic nuclei are discussed. A new schematic picture on the accretion onto a black hole, i.e. an inner hot ADAF and an outer thin disk with corona above, is presented. In Chapter 6 we present our new understanding of the spectral transitions in X-ray binaries in light of the disk evaporation model. Observations show that the Cyg X-1 undergoes occasional transitions between the high and low states. These transitions have been observed for both neutron stars and black hole systems. The two spectral states are thought to be related to different states of accretion, i.e. the one dominated by the ADAF and another by the thin disk. The theory of mass evaporation predicts the formation of an inner hole in the cool thin accretion disk for mass accretion rates below a critical value and a sudden disappearance of this hole when the mass accretion rate exceeds the threshold. The inner edge of the standard thin disk then suddenly shifts inwards from about a few hundred Schwarzschild radii to the last stable orbit. This allows to account quantitatively for the observed transitions between hard and soft spectral states at critical luminosities.